Electron microscopy of defined lengths of chromatin

Defined lengths of chromatin were prepared by brief digestion with micrococcal nuclease and fractionation in a sucrose gradient. A length containing a given number of 200 base pair repeating units appeared as the same number of 100 A beads in the electron microscope. The distance between beads within a length was small, usually less than about 20 A.

An octamer of histones in chromatin and free in solution

Crosslinking with dimethyl suberimidate reveals a chain of histone octamers in chromatin. The octamer can be isolated free in solution at high ionic strength and pH. The identification of dimers formed by crosslinking reveals two or more contacts of each histone with others within the octamer. The molecular weight (110,000) and pattern of dissociation of the octamer are compatible with the composition (F2A1)2(F3)2(F2A2)2(F2B)2.

Measurement of transmembrane potentials in phospholipid vesicles

Phosphatidylcholine vesicles are permeable to tempotartrate, a spin-label derivative of tartaric acid. The inside-outside distribution of tempotartrate is coupled to the inside-outside distribution of H(+), so it must be a measure of the transmembrane electrical potential difference in vesicles permeable to H(+). This prediction is borne out by the finding that the inside-outside distribution of tempotartrate is the reciprocal of the inside-outside distribution of K(+) in vesicles prepared in the presence of valinomycin. The inside-outside distribution of tempotartrate is, by contrast, equal to the inside-outside distribution of Cl(-) in vesicles without valinomycin. This is evidence that an inside-outside Cl(-) concentration gradient induces an H(+) gradient, which must be due to HCl permeation.

Lateral diffusion of phospholipids in a vesicle membrane

Nuclear-resonance spectra of phosphatidylcholine (PC) vesicles are broadened by spin-labeled PC. The broadening of the N-methyl proton line is proportional to spin-labeled PC concentration; the broadening by 1 molecule in 100 of spin-labeled PC is 9.7 Hz at 35 degrees C; the broadening is not due to collisions between vesicles, exchange of spin label between vesicles, or fusion of vesicles. These findings are proof of rapid diffusion in the plane of the PC bilayer (lateral diffusion). A further study of the broadening reveals the molecular frequency of the translation step for lateral diffusion to be much greater than 3 x 10(3) sec(-1) at 0 degrees C.